Tissue hypoxia, or a deficiency of molecular oxygen available for cellular metabolism, is a common cause of death in critically ill people. Tissue hypoxia is caused by innumerable heart and lung diseases and is frequently associated with impaired oxygen delivery to cells and tissues. Within the past century, numerous methods have been developed to attempt to improve oxygen delivery to tissues.
One method of improving oxygen delivery to tissues is through the inhalation of gaseous oxygen through normal respiratory channels. To date, gaseous oxygen may be inhaled through numerous devices such as masks, tents, cannulas, catheters, hoods, and mechanical ventilator systems. In essence, the purpose of oxygen inhalation is to increase the quantity of oxygen absorbed into the blood in hopes of improving oxygen delivery to cells and tissues. However, as a method of oxygen therapy, the inhalation of gaseous oxygen is associated with several complications and limitations. First, if the breathing passages are blocked or if a person has stopped breathing altogether, inadequate amounts of oxygen are absorbed into the bloodstream to sustain cellular metabolism. Second, gaseous oxygen inhalation may be toxic to tissues if inhaled in high concentrations over prolonged periods. Third, the inhalation of high concentrations of gaseous oxygen causes atelectasis or lung collapse.
Other complex methods and processes have been developed to improve oxygen delivery to tissues. These include hyperbaric breathing chambers, extra-corporeal membrane oxygenation (ECMO) or heart-lung bypass machines, intravenous injection of gaseous oxygen, and fluid breathing.
Hyperbaric breathing chambers involve the placement of a person inside a sealed chamber with the subsequent pressurization of the chamber. During chamber pressurization, the patient inhales gaseous oxygen through normal respiratory channels which results in increased blood oxygen levels. However, multiple limitations exist for this method of oxygen therapy. First, the breathing chambers are extremely expensive. Second, complex facilities and highly trained personnel are needed for safe operation. Third, once a person is inside a pressurized chamber, medical personnel outside of the chamber do not have access to the patient. Fourth, since the patient's body is placed inside the chamber, the amount of pressure utilized to pressurize the chamber is limited. Fifth, following chamber pressurization, prolonged time periods are required for the de-pressurization process. Last, the increased blood oxygen levels achieved during chamber pressurization and oxygen inhalation are lost when the chamber is de-pressurized and the person is removed from the chamber. Numerous complications have been documented with this method of oxygen therapy including fires, explosions, oxygen toxicity, gas embolism, and Caisson's disease from rapid chamber de-pressurization.
Another method of oxygen therapy, extra-corporeal membrane oxygenation, involves the removal of the blood from the body, exposing the blood to gaseous oxygen by an oxygenator apparatus, and reintroducing the blood into the body. The limitations of this method for improving oxygenation are obvious when one considers the potential complications associated with removal of blood from the body. First, blood catheters must be surgically inserted within major blood vessels. Second, accidental exsanguination or blood loss, infection, and damage to blood cells are documented complications. Last, this method requires highly trained personnel and is extremely expensive.
Another method of delivering oxygen to tissues which has been tried is intravenous injection of gaseous oxygen. This method has been found to be extremely hazardous. Gas bubbles tend to coalesce in the veins and occlude smaller pulmonary arteries. The resulting gaseous pulmonary embolism causes a decreased pulmonary circulation, arterial hypoxemia, and systemic hypoxia. Due to the extreme hazards, this method of oxygen therapy is generally considered to have little, if any, practical utility.
A relatively new method of oxygen therapy involves fluid breathing. The method was first described by Klystra (1958) who submerged both mice and dogs in a salt water solution inside a hyperbaric breathing chamber. Following pressurization of the chamber with oxygen, the animals inhaled the liquid through normal respiratory channels. The animals remained alive for varying time periods while breathing the liquid solution. Several complications of fluid breathing were noted including increased work of breathing, deficient carbon dioxide excretion, increased fluid retention within the lungs, and grossly impaired respiratory function following the transition from fluid to air breathing.
More recently, fluid breathing by a human infant was reported in 1989 at Hahnemenn Hospital in Pennsylvania. The infant breathed a fluorocarbon liquid which was oxygenated with pure oxygen gas at ambient barometric pressure. The fluorocarbon mixture was used because of high oxygen solubility in the liquid at normal atmospheric pressures. However, liquid ventilation is associated with several complications including decreased blood oxygen levels (hypoxemia), bronchiolar inflammation, wheezing, and carbon dioxide retention. It should be noted that the infant died after several hours of fluid breathing.